This application is the U.S. national phase of international application PCT/GB00/02150 filed Jun. 2, 2000, which designated the U.S.
1. Field of the Invention
This invention relates to rotary pumps.
2. Discussion of Prior Art
Rotary pumps are known devices that are used in a wide range of applications to pump fluids from one place to another and to compress them. A known rot is shown in FIG. 1 of the accompanying drawings. This pump comprises a stator 10 and a rotor 20, the rotor being eccentrically mounted within the stator. The rotor comprises a main body 30 with vanes 40 extending from the main body. The vanes are slideably mounted on the rotor main body such that they can be pushed back into the main body against an outward bias. When the rotor is eccentrically mounted within the stator as shown in FIG. 1, the vanes extend out from the rotor and contact the inner surface of the stator. Due to the eccentric mounting of the rotor the radial extension of each vane varies with angular displacement around the rotor main body.
In operation, rotation of the rotor causes the vanes to sweep along the inner surface of the stator and be pushed back into the rotor main body for the part of the revolution where the rotor main body approaches closer to the stator. The vanes outer rotor surface and stator surface define cavities within the pump. The fluid, for example air, to be pumped enters the pump at the fluid inlet 50. The fluid inlet is located at a point where the rotor is far from the stator, the vanes are extended and the cavity into which the fluid flows is relatively large. As the rotor rotates the vanes defining the input cavity are pushed into the main rotor body and thus the size of the cavity decreases and the fluid is compressed. The fluid outlet 60 is located at a position where the rotor is close to the stator and the vanes are close to or at their minimum extension, thus the cavity is reduced in size and compressed fluid flows out of the fluid outlet An inlet is provided for adding a lubricating fluid such as oil.
In order to prevent fluid leaking from one cavity of the pump to the next the rotor vanes and stator inner liner must provide a seal. This means that the contact between the stator inner liner and rotor vanes must be good and therefore friction between these surfaces tends to be high. A high friction contact between the surfaces results in the rotor being difficult to turn and to wear of the contact surfaces. One way of addressing this problem is to provide lubrication of the surfaces. This can be done by injecting large quantities of a liquid lubricant such soil into the pump. A disadvantage of this approach is that the oil mixes with the fluid as it is compressed by the pump, with several undesirable consequences. The fluid and oil mixture must be separated downstream of the rotary pump, which is an expensive process, the pump must be continually re-lubricated, and pumping the oil in addition to the fluid results in a loss of efficiency.
Oil-free pumps have been provided by coating the moving parts of the pump with a solid lubricant. However, this coating wears away rapidly, producing debris and the need for frequent servicing and replacement.
Page 40 of xe2x80x9cPneumatic Handbookxe2x80x9d, by A. Barber 7th edition, discloses a vaned compressor which has a plurality of floating or restraining rings placed over each vane. The rings rotate with the vanes and maintain a minimum clearance between the vane tips and the casing wall. The rings rotate at a constant speed, whereas the vanes speed varies with extension, so there is some relative xe2x80x9crolling motionxe2x80x9d between vanes and rings. A similar arrangement is disclosed in xe2x80x9cL""air comprime, by J. Lefevre, editeurs Paris, pages 317-318xe2x80x9d. An orbital vane compressor is produced by Dynew Corporation which comprises a bearing mounted within the stator which allows the blades to extend only to a desired amount thereby keeping a clearance with the stator wall.
A further type of compressor is that produced by Robert Groll in co-operation with the company Rotary Compression Systems. This pump has sockets housing sliding vanes
U.S. Pat. No. 2029554 and GB-A-363471 disclose rotary pumps having vanes mounted in pivotable sockets in both the rotor and the rotatable stator inner lining of the pump.
DE-A-4,331,964 discloses a vacuum pump with ball bearings mounted between the stator inner lining and main body.
WO-A-97/21033 discloses a rotary compressor with reduced lubrication sensitivity. In order to combat problems that may occur with liquid lubricants, additional lubrication is provided by adding a xe2x80x9cDLCxe2x80x9d coating to a vane in the compressor. This coating is formed of layer of hard and lubricious substances.
Further examples of other known rotary pumps are shown in British Patents GB-A-2,322,913, GB-A-2,140,089, GB-A-2,140,088, GB-A-809,220, GB-A-728,269, GB-A-646,407, GB-A-501,693 and U.S. Pat. No. 4,648,819.
In accordance with the present invention there is provided a rotary pump comprising: a fluid inlet and a fluid outlet; a stator comprising a main body and an inner liner rotatably mounted within the main body; a rotor comprising a main body eccentrically mounted within the stator; vanes extending from the rotor towards an inner surface of the stator inner liner, the stator inner liner, vanes and outer rotor surface defining pump cavities; wherein the stator inner liner is operable to rotate when the rotor rotates, such that the relative velocity between the vanes and the inner surface of the stator is reduced; the vanes are each mounted such that they are received by and extend between a rotor fixing and a stator inner liner fixing, the motor fixings and stator inner liner fixings being mounted within the rotor and stator inner liner respectively such that the angle of the vanes to the rotor can vary with rotation of the rotor; the rotor fixings and the stator inner liner fixings provide fluid sealing between said pump cavities for normal operation without liquid lubricant, and wherein said vanes and at least one of said rotor sockets and said stator inner liner sockets contact one another at respective contact surfaces, a first of said contact surfaces being a solid lubricant surface and a second of said contact surface being a hard surface so as to provide reduced friction fluid sealing contact without liquid lubricant.
The device of the present invention alleviates the disadvantages of the prior art by providing a stator inner liner that rotates together wit the rotor, thereby reducing the relative velocity between the rotor and stator. This leads to lower sliding speeds and milder contact conditions between the rotor and stator. Thus, the rate of wear of the contact surfaces is reduced. Furthermore, this reduced motion allows the vanes to be held within fixings (such as sockets or bonded bushings) in a manner that allows fluid sealing between cavities without the need for liquid lubricants.
Mounting of the vanes in sockets, results in an improved fluid seal between neighboring pump cavities which gives reduced leakage of pumped fluid between pump cavities. Furthermore, the mounting of the vanes in sockets such that the angle of the vanes to the rotor can vary means that there is no oscillating motion between contact surfaces of the vane tips and stator inner liner with the associated problems of frictional losses and wear of the two surfaces.
Advantageously, the rotor sockets and the stator inner liner socket are rotatable about an axis aligned with their geometric centres and parallel with the axis of rotation of the rotor. In preferred embodiments, the angle of the vanes oscillates about a central position with rotation of the rotor, the central position being preferably with the vanes extending radially outwardly from the rotor.
This is a convenient arrangement that enables the vane angle to change while the rotor rotates while providing a good seal between neighboring pump cavities and reduced frictional wear.
In some embodiments, the vanes are slideably mounted within the rotor socket and are fixedly mounted within the stator inner liner socket.
Although the vanes can be slideably mounted within the socket of the stator inner liner it is preferable that they are slideably mounted within the rotor, as the size of this rotor socket is not restricted by the width of the stator inner liner which is generally quite thin. In order to ensure that the vanes extend to the stator inner liner socket and provide a good fluid seal between cavities, they are fixedly mounted within the stator inner liner.
Preferably, the solid lubricant surface may be PTFE and the hard surface may be one of steel coated with diamond like coatings, tungsten carbide, graphite and molybdenum disulphide.
The rotor, stator inner liner and vanes may be hard coated steel and the sockets may be solid lubricant in the form of PTFE, pure or reinforced with coated glass, bronze, molybdenum disulphide or graphite.
Ball bearings may be mounted between stator and stator inner liner. In this way, the stator inner liner is held in position away from the stator and frictional forces inhibiting rotation are reduced.